Flotation of iron ores



och'l', 1945. E, c; HERKENHOFF 2,387,081 FLOTATI'ONy oF IRON @RES I Filed Dec. lO', 1942 ATTO RN EY Patented Oct. 16, 1945 FLOTATION OF IRON ORES Earl Conrad .Herkenhoil Stamford, Conn., as-

signor to American Cyanamid Company, New York, NL Y., a corporation yof Maine Application December 10, 1942, Serial No. 468,523

6 Claims.

This invention relates to the benecatlon of iron ores by froth flotation. More particularly, the invention-relates to a multi-stage flotation process suitable for use on nes such as are found in the overflow from iron ore washer plants; in

, waste products from heavy media separation processes, and on those ores which must be finely ground in order to unlock the iron minerals from the gangue.

Increasing amounts of the lower grade iron ores, such as those with which the present invention is concerned, are coming into industrial importance because of the constantly decreasing amounts of available high grade ore.` Most of these ores are too low in iron and too high in silica-bearing gangue to be suitable for use in blast furnaces. Consequently, if they are to be used they must be beneciated in some manner. Frequently, calcium carbonate and other alkaline earth carbonate minerals are also found in the ores. These, of themselves, are not particularly objectionable, in fact being to a certain extent desirable. In some cases, however, it may be necessary to remove a portion of these constituents in order to raise the iron content.

The principal problem, therefore, is one of reducing the impurities and raising the iron content to as high a degree as possible. As was pointed out, this usually involves the removal of silica. The necessary degree of beneficiation can be accomplished in any one of several ways, depending on the ore. In some cases, thenecessary vrise in iron content may be made by merely crushing and washing the ore. Where the iron minerals and the gangue are associated in such manner that this can not be done, but the mineral, values can be released with moderately fine crushing, heavy-media separation is particularly useful.

Much of the naturally occurring ore, however, requires ne grinding in order to enable separation of the objectionable gangue. After the necessary grinding is done, beneciation by froth flotation is perhaps the most useful method of separating the gangue from the mineral values. In addition to the ores of this type, there are the wastes from the washing and the heavymedia processes mentioned above. Although too low in iron content for ordinary use, because of the tremendous volumes involved they represent a very large amount of potentially useful iron mineral.

As pointed out above, one of the principal objectionable constituents is usually a silica-bearlng mineral of some type or other. From the point of view of susceptibility to froth ilotation, this is particularly unfortunate since the iron minerals and these silica-bearing gangues are generally difficult to separate. Because of this separational difficulty, a, flotation process to be successfully carried out requires careful handlng, usually also accompanied by a high reagent consumption. All of these factors tend to increase the cost of processing. Yet iron ores, despite the fact that they must be handled in enormous quantities. are intrinsically cheapproducts and the margin of profit is small. Therefore beneciation by froth flotation, if it is to be carried out, must be done cheaply, easily and eiciently. In this respect, ordinary methods of beneciating ores by froth flotation when applied to iron ores leave much to be desired.

Ordinarily, separation of mineral values from silica-bearing gangues by froth flotation takes .one of two forms, Either the mineral is floated from-the silica by the use of an anionic-type reagent or reagent combination, or the silica is floated from the mineral values by means of a cationic-type reagent. In general, anionic flotation is the easiest and cheapest and is', therefore, used whenever possible. The usual procedures are old and well known in conjunction with many ores.

Unfortunately, when applied to ordinary iron ores of the type with which the present invention is concerned, anionic flotation does not work well. The iron minerals and the gaugue tend to float together. By using suilicient care and the necessary amount of reagents a beneciation can be carried out whereby some 30 to 40% of the iron values can be recovered, but Lthe grade is usually too low. The grade can be improved somewhat at the expense of the recovery, but the net cost is much higher than is practically desirable for the amount of mineral recovered at the corresponding concentrations.

In like cases with other ores, the reverse procedure is useful, i. e., silica is floated from the useful minerals with a cationic promoter. This process when it can be properly used is excellent. However, it suffers from several inherent drawbacks which must be capable of being overcome before the procedure can be said to be properly u sed. First of all, cationic reagents are expensive, the unit cost being several times that ofequal amounts of anionic-type reagents. They depend for competitive success on the fact that if they can be used effectively, they are highly selective and have great collecting powers and so can be used in smaller amounts.

However, they must be very eillciently used and this involves a second difficulty, that of slimes.`

Cationic reagents are particularly sensitive to the presence of slimes, even a fraction,v of a percent in the pulp often being suicient tolimpair the` efficiency of the reagent to a pointV Where the reagent cost becomes prohibitive. A `balance must be made betweenl the cost of preparing the ore such as the grinding, desliming, etc., the reagent cost, and the amount of mineral recovered. Unfortunately, with most of the low graderiron, ores this balance Works out unfavorably. The cost of the preparation required plus the reagent cost raises the total above that which can be expended for the amount of ore recovered if the latter is to be sold in a competitive market.

In general, the recovery of iron from low grade ,materials tends to be lower than recoveries of .non-ferrous metals from their ores.:` However, as

previously stated, recovery .is governed by. economic factors which may outweighl metallurgical eiiciency.

There remains, therefore, a demand for a suitable beneciation process whereby the iron content of low grade ores, particularly the wastes from washer plants and the like, can be carried out at a cost which will permit theip'roce-ss tol used on a large scale. It is the object of the prese ent invention to establish a procedure of froth flotation by the use of which the desirable results of cheaper and more eiective concentration of the iron minerals may be obtained with these low grade starting materials.

In general the desirable result is obtained in the present invention bythe use o f a multi-stage flotation process. The ore is suitably prepared so that the bulk of the silica gangue may be floated away from the iron using a relatively cheap anionic reagent. lTailings from this flotation contain the bulk of the iron minerals and after being deslimed and washed are subjected to a second flotation in which enough of the residual silica content is removed by means of a cationic reagent to give the desired grade.

The invention may be illustrated in conjunction with the accompanying dra-wing in which:

Figure 1 is a flow scheme showing the essential steps in the process, and

adapted to give slightly higher recoveriesr additional frothing agent may be useful. A1- though it is usually preferable to carry out this cleaning step by flotation, other methods such as gravity concentration, as for example;- tabling or vanning, may be employed with-advantge on certain types of ores. As shown ini Figure l', the tailing from the primary silica flotation is subjected to a desliming operation. It may be desirable to subject the primary silica concentrate to a cleaner flotation as shown for example in Figure 2. In this case the tailing from the cleaner' notation is combined with the rougher tailing from the primary silica flotation before being deslimed. Ordinarily the slimes are discarded as shown in Figure 1. may be recycled to the alkaline-earth-oxide conditioning stepas shown in Figure 2.

A cationicreagent, usually accompanied by a frothing agent, is added to the deslimed sands As shown in Figure l, the oreislnormallygfed 1 I,

to the process in a size range suitable `for use in'r flotation feed. In the case of washer tailsand the like, no crushing or grinding of the ore by way of preparation is ordinarily required. If previously untreated ore is used, it may require grinding to a suitable size. With some feed, a desliming step such as is shownin Figure 2 may' be usefully employed. TIn deslmii'ig, the use. of

,andthe bulk of `the `'residual silica oated out. Again, the silica eoncentnate may he directly discarded orzmay be cleaned` `lnhelatter case the cleaner silica concentrateiriay be discarded and the cleaner tail recycled "to the alkaline-earth oxide c'z'onditionirig step. The rougher tail constitutes the product and may be sold as such.

.A number of modifications may be made without departing from the scope of `the present invention. For: example, the grindingstepmay be carried out after the feed material 'has been deslimed. Again, where the desliming step is not necessary the grinding operation may be made to serve as the first conditioning step, the grinding being carried out in the presence of a dispersant and the aionic reagent which is to be incorporated. As poined out, each flotation operation may be varied somewhat and the various cleaning operations may be done either by flotation or by gravity concentration or may not even be required. These, however, fall within the skill of the operator and may be varied according to conditions for the particular ore being treated.

An anionic reagent is required in the primary silica flotation step and a cationic reagent in the secondary notation step, otherwise the process is not necessarily limited as to the nature of the reagents used. In-the anionic flotation substantially any of the common non-sulfide promoting notation reagents of the fatty-acid type may be used suchas, for example, oleic acid, fish oil fatty` acid, coconut oll fatty acid; linseed oil `fatty acid cottonseedoil fattyv acid, talloel andthe vlike asVA desirable to use', because of its cos'to'r availability- #will lgives'at'isfa'ctory'resultsi .Amongrthe frothers a dispersinga'gent, such; for'examplas l stadiums-o0 silicate or ,thejiilt isf'or'dinarily used to facilitate the separation; n

The whole ore or the sandsissubjected two successive conditioning steps. In the first-of these, the ore is treated with the anionic reagent to be used in the subsequent notation. In the second, an alkaline earth metal oxide or hydroxide1 such as hydrated linie or the like is incorporated'l to serve as a depressant for the iron minerals. A frother is then added and the material subjected to a flotation st'p whereby a concentrate high in silica and low in iron is obtained.

The silica concentrate from the anionic flotation may be either discarded or be cleaned. If cleaning is done by flotation a small amount of [which wereffound to Lbe `satisfactoryare such materia-leas `pine oil, synthetic-:pine 'oilgfcresylic acids,

:fthe commercial aliphatic alcohol frothers and mixtures'fof these with .each other or with various modifying agents.

Again, in the secondary silica notation the particular cationic reagent chosen is dependent upon relative cost and availability. Among the reagents found suitame are various aliphatic amines naving a carbon content of over about C12 and their salts; Quaternary onium" compounds such as cetyl pyridinium bromide, for example; and the polyalkalene-polyamine r e a c t i o n products. Again, these compounds are illustrative only since many known compounds are commercially available for this purpose.

However, if so desired they 2,387.081 A particularly important feature of the presentprocess is the fact that the ore, prior to the anionic flotation step, is conditioned with an alkaline earth metal oxide or hydroxide. The exact mechanism by which this reagent accomplishes its desired function is notZfully understood. However, it appears to act in such a vmanner as not only to act as a depressant for the iron minerals but also to assist the selective flotation of silica during the subsequent notation with the anionic reagent. Hydrated limewas found to give excellent results and since it is cheap and readily availabl'e is perhaps preferable. However, any of the alkaline earth metals which tends to form an insoluble soap with the fatty acid component of the anionic reagent may be used in the form of their oxides or hydroxides, if so desired. It would appear vthat it is not only necessary to add the oxide or hydroxide 1for the ypurpose of controlling the pH :content of the pulp but the fact that the alkaline earth metal hydroxides tend to form insoluble limportance. Y The invention will be describedy ingreater detail in cbnjunction 'with `the *following speci'c `oaps is for some "reason .of equal -i not greater examples which are illustrative only and arenot meant to limit the scope of the present invention.

EXAMPLE` 1 v A sample of a siliceous--hem'at'iteiron'ore was ground in a wet rod mill at about 60% solids in the presence of about 2 lbs. per ton of sodium silicate and 1.5 lbs. per ton of talloel. The ground ore was transferred to a Fagergren flotation ma'- chine, diluted to about 23% solids, conditioned for 3 minutes with 4 lbs. per ton of hydrated lime and floated for 5 minutes.

Table I ,"Pereent Percenti'v r`*PercevntwVv I 4 v weight vFe distribution f 100. 00 28. 80 100. 00 s 56. 72 19. 27 37. 95 Slimcs 5. 16 v454i. 11 8. 62 Secondary silica conc 17. 20 22. 52 13. 85 Pro uct 20. 86 54. 67' 39. 5S

EXAMPLE 2 A sample of an ore similar to that used in Example 1 was ground at 60% solids for 12 minutes using 2 lbs; per ton of sodium silicate as adispersant. The ground ore was .then subjected'fto a desliming operation by hydraulic classification.l The slimes were discarded and the deslimed sands subjected to a double conditioning step. In the first step the sands were conditioned for 3 minutes with 0.5 lb. per ton of talloel and in the second step with 2.0 lbs. per ton of hydrated lime. 0.054 lb. per ton of pine oil was then added and the pulp floated for 4 minutes. The silica concentrate was cleaned using 1 lb. per ton of hydrated lime 'and 0.027 lb. per ton of pine oil, the silica cleaner concentrate being discarded. The combined rougher and cleanen tails Were again deslimed. the slimes .being discarded and theunderfiow subjected to flotation for 3 minutes at 13% solids in the presence of 0.189 lb. of pine oil per ton and 0.15 lb.of n-octadecylamine. The silica concentrate was discarded and the tailing constituting By way of comparison with Example- 2 a sampleof a similar ore was ground,`deslimed, conditionedwith a sufiicient arno'untfof the same cationic reagent to give the maximum recovery and grade, the amountbeing determined by experiment. Using several times the amount of cationic reagent required for the procedure of Example 2, an iron concentrate was obtained representing 18% by weight of the original ore,assying 56.32% iron and containing 35% of the total iron. The Vmuch less expensive procedure of the present invention therefore gave a substantially higher recovery at a substantially equivalent grade.

EXAMPLE 4 A sample of tailings from a Minnesota iron ore washer plant principally composed of hematite and quartz and containing about 19.8% iron was deslimed with 1 lb. per ton of sodium silicate as a dispersant. The deslimed sands were then treated according to the conditioning and flotation procedure of Example 2 using oleic. acid as the anionic reagent and Lorol amine hydrochloride as the cationic reagent. The results are shown in Table III.

Table III Percent Percent Percent Weight Fe distribution 10o. oo 19. 85 100.00 19. 21 18. 71 18.11 35. 20 8. 24 14. 61 1. 20 18. 47 1. l2 Secondary silica conc 25. 22 11.35 14. 42 Pro uct 19.17 53. 57 51. 74

Comparison of these resultse with the results obtained using the best single stage flotation procedure and employing cationic notation reagents.

indicates that .by thel process of the present inventionleven a low grade feed such as the washer wastes` of Example 4, yields a greater'quantity of concentrate; of satisfactory grade. This concentrate is notgjonly obtained from a poorer feed but was obtained'at a much smaller cost. While the concentrate is two points less in grade, it represents a. much greater proportion of the total iron content inthe feed.l

EXAMPLE 5 during the rst conditioning stage in addition to the other reagents. The results are shown in Table IV.

1. A method of beneciating iron ores contain-` ing a silica-bearing gangue by froth flotation which comprises the steps of making a pulp of the ore of a size suitable for use as flotation feed and subjecting the pulp to two successive conditioning operations in the rst of which an anionic promoter selected from the group consisting of the higher fatty acids, resin acids, talloel, and the sodium, potassium and ammonium'soaps thereof, is incorporated and in the second of which a suflicient amount of an alkaline earth metal oxide to depress the iron minerals is incorporated, subjecting the conditioned pulp to a froth flotation operation, subjecting the resultant tailings to a desliming operation and subjecting the deslimed sands to a froth otation operation in the presence of a cationic-type promoter, whereby a tailing rich in iron and low in silica is produced.

2. A method according to claim l characterized in that the ore is ground to a size suitable for notation and deslimed subsequent to the grinding operation.

3. A process according to claim l characterized in that the ore is ground to a size suitable for flotation and the rst conditioningl operation is carried out simultaneously with the grinding.

4. A process according to claim 1 in which the alkaline earth metal oxide comprises calcined lime.

5. A process according to claim 1 characterized in that the secondary silica concentrate is cleaned, the secondary silica cleaner concentrate being passed to waste and the secondary silica cleaner tail being recycled to the original conditioning step.

6. A method according to claim l characterized in that the slimes from the desliming step which follows the primary silica flotation step are recycled to the alkaline earth metal hydroxide conditioning step.

EARL CONRAD HRKENHOFF. 

